"Conducted the funeral service" puts it mildly. The account linked above says Helmer was killed on the morning of May 2 when he "left his dugout and was killed instantly by a direct hit from an 8 inch German shell. What body parts could be found were later gathered into sandbags and laid in an army blanket for burial that evening." No chaplain was present, so McCrae "conducted a simple service at the graveside, reciting from memory some passages from the Church of England's 'Order of Burial of the Dead'."
Helmer's grave was marked, but the marking eventually was lost, and he "he is one of the 54,896 soldiers who have no known grave in the battlefields of the Ypres Salien."
In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.
We are the Dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved and were loved, and now we lie
In Flanders fields.
Take up our quarrel with the foe:
To you from failing hands we throw
The torch; be yours to hold it high.
If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.
The last stanza charges future generations to "take up the quarrel against the foe," and we can and should consider carefully who/what that foe is and how best to "take up the quarrel." Nevertheless, as long as we live in an imperfect world where evil vies for our hearts and minds and lives and nations, all of us are called to protect, to guard, to defend -- whether through military service or through civil service or through active, thoughtful citizenship -- the ideas* that good exists, that good will ultimately prevail, and that--until then--it is better to die, physically or in the small daily sacrifices we make for others, in the service of good than it is to pretend that evil does not exist, that the conflict does not exist, and/or that we are not called to serve.
None of us is innocent. None of us is excused. None of us ever can be said to have retired from active duty. We each have been thrown a torch. Will we take what is good and use it for evil to burn, kill, and destroy? Will we, out of fear that we are misusing it, stamp it out and, having blinded ourselves to good, stumble about in evil's darkness? Or will we hold it high to shed as much good light as we can so all can see to work for good.
*When a recruit joins the U.S. military, he/she takes an oath to "support and defend" what? The president? The country? The people? Read the oath here. Here is another Department of Defense article titled "Why Civilian Control of the Military?" -- worth reading.